Abstract: Utilizing self-designed experimental device to exert ultrasonic vibration on the workpiece in front of the stirring tool,the effect of ultrasonic vibration on reducing weld defects, improving the microstructure and mechanical properties of friction stir welded joint was investigated experimentally. A series of tests on ultrasonic-vibration-enhanced friction stir welding were conducted on 6 mm thick 6061-T4 aluminum alloy sheets. The experimental results were compared with those by conventional friction stir welding under the same welding conditions. The results show that ultrasonic vibration could reduce the tunnel defects caused by a high ratio of welding speed to rotation speed, enlarge the width of mixed zone and the volume of weld nugget, refine the grains in the weld nugget and thermo-mechanical affected zone,and improve the tensile strength of joints and the microhardness in the weld nugget.